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Re: Reversed Hallux in Theropods: a query



GO writes:

>The trouble is, you've seen it reconstructed only in cursorial theropods, in
>which it was invariably reduced and repositioned. But if you want to see what
>the retroverted hallux really looked like in the arboreal forms, take a look
>at Luis Chiappe's paper on his Patagonian enantiornithan bird specimen
>(sorry--exact reference is not handy, but check out any of his most recent
>bird-paper reference lists and you'll find it). The hallux ungual is
>_markedly_ larger than the unguals of the other toes, and it points forward,
>in clear opposition to the other toes. This was a grasping foot, no question
>about it. It is almost difficult to imagine, in fact, how this bird could
>have walked with a claw like that stuck on its "heel."

Without seeing the reference: there are quite a number of modern birds  with
large, strongly-curved ungual phalanges that get about on the groulnd quite
well (eg Australian treecreepers (Climacteris) which have a very strong
hallux and ungual claw used to support them when climbing but also forage on
the ground.  Quite a few terrestrial larks (to name one example) have the
hallux ungual far larger than the others, and though it is straight in some
species (eg the Spike-heeled Lark Chersomanes albofasciata) it is strongly
curved in others.  Yet these species are almost if not entirely terrestrial.
So are some species of woodpecker.  Yes, surely these birds evolved from
arboreal ancestors, but they are perfectly able to get about on the ground.

>
>But in all theropods, cursorial or not, that retained metatarsal I, it
>articulated with the _back_ of metatarsal II, which even had a shallow groove
>as the joint. There is a fair amount of rotation that the tip of metatarsal I
>could undergo within the groove, so some liberty is used when making drawings
>of that region of the skeleton, and digit I can be turned out sideways and
>even pointed forward.

The modern osprey can do this too, as can swifts and Mousebirds (Coliidae).
The ability to rotate the hallux outward and forward is particularly marked
in certain woodpeckers (the Ivory-billed Woodpecker was an example); in this
case, of course, the opposing mechanism to the clinging foot is the tail.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
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